It is currently possible to increase or "upgrade" computer memory using memory modules. An example of a memory module is a single in-line memory module or "SIMM." A SIMM is typically comprised of a printed circuit board with at least one memory component such as a RAM, SRAM, DRAM, and the like attached thereto. The SIMM also commonly includes a multi-pin/pad single in-line connector for connecting the SIMM to the device being upgraded. A detailed description of such a SIMM is found in commonly owned U.S. Pat. No. 5,272,664 to Alexander et al., filed Apr. 21, 1993 entitled "High Memory Capacity DRAM SIMM." U.S. Pat. No. 5,272,664 is incorporated herein by reference.
Prior Art FIG. 1 is a side cut away view of a pair of SIMMs 10 and 12 conventionally mounted to a mother board 14. Each of SIMMs 10 and 12 include a memory component 16 and 18, respectively. Memory components 16 and 18 are attached to substrates 20 and 22, respectively. SIMM 10 is attached to mother board 14 via edge connector 24. Similarly, SIMM 12 is attached to mother board 14 via edge connector 26. A processor 28 with a heat sink 30 coupled thereto is bonded to mother board 14.
As shown by Prior Art FIG. 1, an extended bus is required for processor 28 to access memory components 16 or 18. That is, in the prior art, signals traveling from processor 28 to SIMM 10, for example, must travel from processor 28 along an address line present in mother board 14. The signals must continue to travel along the address line through mother board 14 and to edge connector 24 up into substrate 20. The signals then travel along a cache bus in substrate 20 of SIMM 10. The cache bus extends from edge connector 24 to memory component 16.
In the prior art, component layouts generate long "routing distances" between the processor and SIMMs thereby necessitating such extended address line lengths. As shown in Prior Art FIG. 1, heat sink 30 extends beyond the periphery of processor 28. Thus, heat sink 30 prevents SIMMs 10 and 12 from being placed directly next to processor 28. It will be seen from Prior Art FIG. 1, that memory components 16 and 18 are separated from mother board 14 by at least the height of edge connector 24. Thus, the cache buses must extend at least from the memory component and through the vertical length of the edge card connector. Additionally, as processors generate greater quantities of waste heat, heat sinks become larger to dissipate the waste heat. As a result, prior art SIMMs are pushed farther from the processor by the protruding edges of the ever-larger heat sinks. Thus, prior art address line lengths and cache bus lengths are extended. Furthermore, passing signals form the mother board to the SIMM via the card edge connector creates electrical continuity problems.
Increased address line run lengths and cache bus lengths also inhibit the speed at which signals are effectively transferred. More specifically, as address lines and cache buses lengthen, transmission line effects increase. In the prior art, extended address line and cache bus lengths generate transmission line effects which limit effective signal speeds to well under 200 MHz. Furthermore, simultaneous switching noise generated as a result of the numerous outputs from prior art SIMMs to the mother board creates tremendous transient current. The transient current passes through parasitic inductance and generates noise on ground. Noise on ground is highly undesirable because all voltage signals are referenced to ground. Although prior art SIMMs have several limitations, the SIMMs still possess substantial utility. As an example, the "socketability" or replaceability of the SIMMs is an advantageous feature. Thus, even if substantial improvements are made over existing prior art memory modules, socketability features should be retained.
Thus, a need exists for a memory module which does not require extended address lines or cache buses, a memory module which is capable of operating at high speeds up to 200 MHz, and a memory module which is socketable.